It is possible for flowing water, and possibly other liquids, to carry radioactive materials, such as radioactive nuclides. This may occur in the case of water discharged from a nuclear reactor installation but may be involved in other instances, such as industrial waste water in general. By continuously monitoring the radioactivity of the flow, the radioactivity can be kept within acceptable limits, such as by taking corrective measures at the source of the flow.
Such monitoring has been done by the use of a flow cell through which a stream of the water flow is passed and into which a beta or gamma sensitive detector is inserted, the detector being of the type which produces pulses which are fed to a conventional pulse counter which activates an alarm if the pulse rate exceeds one that is predetermined as indicating an upper limit of safety. The pulse rate is, of course, proportional to the radioactivity of the water stream.
The response probability of such equipment and the zero rate of the detector is, as a generality, about several 10.sup.-.sup.7 .mu.Ci/ml; the different radiation properties of the radioactive nuclides carried by the stream may show greater or lesser deviations from such a value. It is desirable to increase such a response probability which in practice is often substantially less favorable because of contamination of the cell, such as in the form of deposits on the detector which cannot be avoided since the cell must be continuously in service for long time periods.
There are instances when the concentrations of radioactive nuclides or radioactive particles are very low and cannot be monitored satisfactorily in the described manner. For example, a water-cooled reactor may feed the heat-exchange tubes of a steam generator and the steam condensate may ultimately be discharged as a water flow. The heat exchanger comprises a multiplicity of tubes, and if one of these tubes leaks even slightly, the flowing water discharge may carry a very low concentration of radioactive material.
Heretofore the only way to measure such very low reactivity has been by taking samples of the water and measuring them in a laboratory, possibly after chemical concentration of the radioactive nuclide. In this way, detection limits in the order of 10.sup.-.sup.9 .mu.Ci/ml and less can be obtained, but for practical reasons this practice is limited to the testing of random samples of the water flow. Between the sampling the actual radioactivity of the flowing water remains unknown and this may represent extended time periods.
The object of the present invention is to provide a method and apparatus for continuously monitoring flowing water, and possibly other liquids, when the radioactivity is so low as to be outside of the capability of the previously described continuous monitoring method and equipment to be used effectively.